There has been practically used a microwave landing system (simply called "MLS" hereinafter) which includes an azimuth station scanning in the horizontal plane and an elevation station scanning in the vertical plane. In the MLS, the coverage region is generally 40 degrees respectively on both left and right sides in regard to the center line for azimuth and 0.9 to 20 or 30 degrees for elevation. The scanning beam is narrow in the direction of scan, typically 1 to 3 degrees for azimuth and 1 to 2 degrees for elevation.
In operation, the beam is scanned in the coverage region from one scan limit to the other and then back. That is to say, the horizontal region is covered by the beam which scans from the left scan limit to the right scan limit (called "TO scanning") and then back to the left scan limit from the right scan limit (called "FRO scanning"), while the vertic al region is covered by the beam which scans from the lowest elevation limit to the highest elevation limit (called "TO scanning") and then back to the lowest elevation limit from the highest elevation limit (called "FRO scanning"). During each scan cycle, an aircraft receives TO and FRO pulses in accordance with the TO and FRO scannings a time interval between which is in response to a position in the air at which the aircraft is flying to make a landing on a runway so that the measurement of the time interval provides the information of angles for azimuth and elevation. The MLS as mentioned above is referred to as Time Reference Scanning Beam System (TRSB) which is well known in the art, while the details thereof are described later.
According to the MLS, however, there occurs multipath propagating beams in accordance with the reflection of the scanning beam or sidelobes thereof on the ground or reflecting medium like buildings etc. This results in multipath errors in the measurement of a time interval between the TO and FRO pulses. In order to reduce the multipath errors, an aperture of an antenna must be enlarged. In addition, such hardwares as radiating elements, phase shifter, transmitting line etc. are difficult to be manufactured because they are required to have a predetermined quality. Further, high frequency circuits of complicated construction are necessary to be added for forming a spatial filter.